Electrical energy can be harvested or generated using a variety of techniques. For example, an energy source can include a mechanical-to-electrical conversion device such as a piezoelectric energy harvesting (PEH) device. Another example of an energy source can include a photovoltaic (PV) device, such as a solar cell or a solar array. Yet another example of an energy source can include a thermo-electric generator (TEG). Use of a PEH device, PV device, a TEG, or other energy sources can include coupling such an energy source to a load to provide operating energy for the load. However, there can be a detrimental effect on energy conversion efficiency or power transfer efficiency if the load current or voltage requirements are badly mismatched to the output characteristics of the energy source.
FIG. 1A illustrates generally an example including an equivalent circuit that can model an energy source such as a piezoelectric energy harvesting (PEH) device. The example 100A of FIG. 1A includes a circuit portion corresponding to a mechanical behavior of the PEH device (“MECHANICAL”), and a circuit portion corresponding to an electrical behavior of the PEH device (“ELECTRICAL”), such as corresponding to a bimorph mechanical configuration that can include a cantilever structure. The mechanical portion of the PEH device can be represented as a mechanical spring-mass-damper system, where LM can represent the mechanical mass, CM can represent the spring stiffness, and RM can represent a damping effect (e.g., mechanical loss), excited by an input mechanical force represented by σIN. A current, i, can be provided in response to the input force, such as provided using a model including a coupled inductor structure (e.g., a transformer) having a turns ratio, n. The transformer can represent an element bridging the mechanical and electrical domains.
An electrical portion of the PEH device structure can be modeled as a parallel capacitance, CP, coupled between the output nodes. In generally-available PEH devices, an amplitude of the AC current source is dependent on the acceleration level (e.g., “g” level) induced by vibration. The parallel capacitance CP is generally on the order of about tens of nanoFarads (nF) to hundreds of nF.
In one approach, a PEH device can be electrically coupled to a rectifier circuit. The rectifier circuit is generally coupled to an energy storage element and a load that draws energy from the storage element. The PEH device converts vibrational mechanical energy into a time-varying electrical signal. The time-varying signal is rectified and energy recovered after rectification can be stored into the storage element to provide a supply voltage for a load circuit, such as including a downstream power conversion or regulator circuit. Generally, the PEH device has a non-negligible parasitic capacitance that traps energy. Accordingly, an operating point for maximum power transfer might not be established exclusively using a resistive matching technique. For steady-state operation where the PEH device provides an alternating current (AC) waveform having roughly constant amplitude, a conjugate matching technique can be used such as to compensate for energy trapping occurring in the reactive component of a PEH device source impedance.